Physics A Level
Chapter 26: Electromagnetic induction 26.1 Observing induction
Physics A Level
Chapter 26: Electromagnetic induction 26.1 Observing induction
GENERATING ELECTRICITY
Most of the electricity we use is generated by electromagnetic induction. This process goes on in the generators at work in power stations, in wind turbines (Figure 26.1) and, on a much smaller scale, in bicycle dynamos. It is the process whereby a conductor and a magnetic field are moved relative to each other to induce, or generate, a current or electromotive force (e.m.f.).
One of the most important principles in physics is the idea of conservation of energy. You cannot just produce electrical energy from nowhere. In the case of a generator or a dynamo, how is the electrical energy produced?
You can carry out some simple experiments to observe features of electromagnetic induction. These are described in Practical Activity 26.1.
PRACTICAL ACTIVITY 26.1: OBSERVING INDUCTION
For each experiment, try to predict what you will observe before you try the experiment.
Experiment 1
Connect a small electric motor to a moving-coil voltmeter (Figure 26.2). Spin the shaft of the motor and observe the deflection of the voltmeter. What happens when you spin the motor more slowly? What happens when you stop? Usually, we connect a motor to a power supply and it turns. In this experiment, you have turned the motor and it generates a voltage across its terminals. A generator is like an electric motor working in reverse.
Experiment 2
Connect a coil to a sensitive microammeter (Figure 26.3). Move a bar magnet in towards the coil. Hold it still, and then remove it. How does the deflection on the meter change? Try different speeds, and the opposite pole of the magnet. Try weak and strong magnets.
With the same equipment, move the coil towards the magnet and observe the deflection of the meter.
Experiment 3
Connect a long wire to a sensitive microammeter. Move the middle section of the wire up and down through the magnetic field between the magnets (Figure 26.4). Double up the wire so that twice as much of it passes through the magnetic field. What happens to the meter reading now? How can you form the wire into a loop to give twice the deflection on the meter?
Factors affecting induced e.m.f
In all the experiments described in Practical Activity 26.1, you have seen an electric current caused by an induced e.m.f. In each case, there is a magnetic field and a conductor. When you move the magnet, or the conductor, there is an induced e.m.f. When you stop, the current stops.
From the three experiments, you should see that the size of the induced e.m.f. depends on several factors.
For a straight wire, the induced e.m.f. depends on the:
- magnitude of the magnetic flux density
- length of the wire in the field
- speed of the wire moving across the magnetic field.
For a coil of wire, the induced e.m.f. depends on the:
- magnitude of the magnetic flux density
- cross-sectional area of the coil
- angle between the plane of the coil and the magnetic field
- number of turns of wire
- rate at which the coil turns in the field.